The present invention relates to translation circuits which respond linearly to logarithmic changes in input signals and, more particularly, to a wide-range logarithmic responding circuit which can be used to provide a voltmeter transducer analyzer which is sensitive to input signals which may vary over a 10 decade range.
Transducer analyzing devices such as voltmeters which respond linearly to decibel changes in input signals are known and have been widely used in the art. These prior art voltmeters are of limited usefulness, however, as their dynamic ranges are particularly limited, e.g. 40 to 60 db. Further, the sensitivity of these prior art voltmeters is typically limited to one millivolt; and in order to extend the dynamic range of these voltmeters to levels below one millivolt, an additional or "outboard" preamplifier is required.
These prior art voltmeters are typically arranged to first rectify the incoming signal to provide a "one-quadrant", unidirectional or D.C. interim signal, and thereafter a D.C.-coupled, one-quadrant logarithmic converter or amplifier is used to provide an output signal to a display device or meter. In addition to having limited sensitivity, these prior art voltmeters suffer with several disadvantages. For example, due to the use of DC-coupled circuitry, compensation circuitry for component and temperature variations is necessarily sophisticated and complex; the dynamic range is limited to a range of about 40 to 60 db maximum. Further, when the input signal is removed from the voltmeter, the output display meter either fluctuates wildly or its moving member remains pinned against the left meter stop; overall frequency response is limited as each voltmeter stage must exhibit very wideband frequency characteristics if the cascaded combination of stages is to be wideband; and, input signal levels below approximately one millivolt require external preamplification.
These and other disadvantages are overcome by the present invention wherein the input signal is first A.C. logarithmically amplified and converted; thereafter, the A.C. output of the converter is rectified and then applied to the output display device or meter. Accordingly, in accordance with the present invention, only one stage need be DC-coupled; D.C. rectification of the converter output signal is relatively simple as the levels thereat are relatively high; stringent frequency response considerations are primarily applicable to only one stage, i.e., the A.C. logarithmic converter stage; and, overall accuracy of the output indication is largely dependent only on one stage, again the logarithmic conversion stage. Further advantages provided by the present invention are that relatively simple circuitry can be deployed, and very low power drain is attained.